CODEWITHSUNDEEP
javascriptes6-promise
ikzjfr0
ikzjfr0

Reputation: 767

general solution to retry a promise in javascript

I try to give out a general solution for retrying a promise. Below is my way and comes an error of "Uncaught (in promise)".

How can I fix this problem? 

function tryAtMost(maxRetries, promise) {
  let tries = maxRetries

    return new Promise(function(resolve, reject) {
        promise.then((result) => {
        resolve(result)
      })
        .catch(err => {
            if (tries > 0) {
                console.log(`tries with ${tries}`)
          tryAtMost(--tries, promise);
        } else {
          reject(err)
        }
        })
    })
}

tryAtMost(3, promise).then(result => {
  console.log(result)
})
.catch(err => {
  console.log(err)
})

Upvotes: 2

Views: 2828

Answers (1)

T.J. Crowder
T.J. Crowder

Reputation: 1074335

The reason for what you asked about is that you're missing a call to resolve in your catch:

.catch(err => {
    if (tries > 0) {
        console.log(`tries with ${tries}`);
        resolve(tryAtMost(--tries, promise)); // <=== ****
    }
    else {
        reject(err)
    }
})

...and so you're creating a promise that is never handled by anything, and so if it rejects, you'll get an unhandled rejection.

But, tryAtMost has a problem: It cannot work with the information you've given it, because it doesn't know what to try. You need to pass it an executor, not a promise, because it's the work of the executor that you need to retry. This also makes the function a lot simpler:

function tryAtMost(tries, executor) {
    --tries;
    return new Promise(executor)
        .catch(err => tries > 0 ? tryAtMost(tries, executor) : Promise.reject(err));
}

Use:

tryAtMost(4, (resolve, reject) => {
    // The thing to (re)try
});

Example:

function tryAtMost(tries, executor) {
  console.log(`trying, tries = ${tries}`);
  --tries;
  return new Promise(executor)
    .catch(err => tries > 0 ? tryAtMost(tries, executor) : Promise.reject(err));
}

tryAtMost(4, (resolve, reject) => {
  const val = Math.random();
  if (val < 0.3) {
    resolve(val);
  } else {
    reject(val);
  }
})
.then(result => console.log(result))
.catch(err => console.error(err));

Upvotes: 5

Related Questions